Anisotropic spinning solutions from aromatic polyamides have been described in Kwolek U.S. Pat. No. 3,671,542 and U.S. Pat. No. Re. 30,352. These solutions (dopes) are useful in making aramid fibers of very high tenacity and modulus.
Optically anisotropic solutions of cellulosic materials have been described in French Pat. No. 2,340,344, and these too have provided high tenacity/high modulus fibers. The ever-increasing costs of petrochemicals gives increasing impetus to the study of fibers from renewable sources, such as the cellulosics. In particular cellulosic fibers with properties approaching the aramid properties have been sought. Considerable effort has been applied to the use of optically anisotropic solutions to obtain the desired properties.
In the cellulose textile field it has been proposed that higher DP (degree of polymerization) should provide improved properties in the resulting fibers or films but it has not been possible to accomplish this goal because of the extremely high viscosity of the solutions. Anisotropic solutions provide the opportunity for spinning at high concentrations without excessive viscosities.
Dissolution of cellulosic polymers can be quite rapid in strong inorganic acids like sulfuric acid, nitric acid, phosphoric acid, and hydrochloric acid and severe molecular weight loss and competitive esterification reactions have rendered such solvent systems of limited utility for the preparation of high performance shaped articles. U.S. Pat. Nos. 1,521,876 (Farrow), 1,943,461 (Traill), and 4,370,168 (Kamide) are illustrative of those describing the utility of a variety of aqueous inorganic acids in degrading cellulosics to lower molecular weight or to alter the type and distribution of substituent groups on the backbone through hydrolysis or esterification.
U.S. Pat. No. 4,370,168 further describes cellulose derivative materials dissolved in an aqueous solution of inorganic acid, but prior to the present invention, adequate inorganic solvents for forming high concentration solutions of high DP cellulose triacetate have not been available. Additionally, the solvents of this invention give rise to cellulose triacetate mesophase solutions which are uniquely resistant to chain scission and substituent hydrolysis. This enhanced stability is the result of using cellulose triacetate, high solids solutions and the greatly decreased hydrolyzing power of the specific solvent mixtures described. The ability to dissolve and maintain high polymer molecular weight and prevent esterification by the nitric acid in high concentration mesophase systems is an essential feature and has resulted in the preparation of fibers or other shaped articles with far superior mechanical properties versus those described previously from inorganic solvents.